The eggs are denting from one of two reasons: a.) It isn't fertile, or it went bad at some point or b.) there isn't enough moisture. Also, if your temps are fluctuating between 84 - 90 degrees, that's way too much of a range. The temperature the eggs stay at needs to be pretty consistant, or you run the risk of dealing with severe deformities. You need a good proportional thermostat to keep the temperature steady. I use one by Helix, and it was pretty expensive, but it keeps my 'bator very consistant.
This thread explains a great procedure that I use to incubate my leo eggs.
http://www.albeysreptiles.com/incubate-eggs.htm Several other breeder friends of mine use this technique as well. If you're using vermiculite, you need to use 1 part vermiculite to 1 part water. The ratio is a bit higher with vermiculite. (So if there's 60 grams of vermiculite, add 60 grams of water to make a total of 120. ) If you're STILL seeing denting while you're incubating the eggs following Albey's method, you can put a damp paper towel on top of them and see if that helps plump them up. NEVER spray the eggs directly. And ya know, sometimes they're just gonna dent. Last season I had a really s****y season, and that kept happening to me as well.
Get the second tank as soon as possible. My best suggestion would be to keep two in one tank, and two in the other. Even though you're not seeing outwardly signs of stress, they still might be, as leopard geckos are terrific at adapting to whatever surrounds them to be able to survive.
You need to be able to deal with birth defects and deformed hatchlings, and hatchlings that are dead in their eggs. You also need to have a good vet on hand, who you will frequently see, when you need fecal samples analyzed and when medical problems arive.
You also need to plan to keep all the babies yourself if something happens, and your friends decide they don't want the gecko.
Tanks aren't that expensive, they're only around $20 or $30 bucks or so at one of the large chain stores. If you can't afford that, how can you afford the expense of raising hatchlings (which is MUCH more expensive)?